1. Field of the Invention
The invention relates to turbomachines, particularly but not exclusively to turbomachines for aircraft, and more specifically to turbomachines including at least one device which bleeds gas from the compressor of the turbomachine to provide a supply of pressurized gas. The invention also relates to the application of this device to the cooling of the rotor of the turbine of the turbomachine.
2. Summary of the Prior Art
Turbomachines are well known machines which essentially comprise a rotary compressor with vanes, a combustion chamber and a turbine, the stream of gas which passes through them from the upstream end to the downstream end being subjected to an appropriate thermodynamic cycle. Turbomachines are used in particular in aeronautics for aircraft propulsion, and are also used in industry. They often include at least one device for supplying pressurized and somewhat cold gas intended for various uses, such as cooling parts such as the turbine, controlling operating clearances, heating the fuel or the lubricant, and aircraft auxiliaries.
Such a device for supplying pressurized gas customarily includes means for bleeding gas from the compressor at a desired pressure, means for cooling the gas thus bled to a desired temperature, and means for conveying this gas to the items which will use it.
The means for bleeding the gas from the compressor comprises at least one bleed orifice opening into the gas stream and arranged at a point in the compressor where the pressure has reached a sufficient level.
The means for cooling the bled gas essentially comprises a heat exchanger connected to the bleed orifice by a pipe. The heat exchanger consists of a cooling cavity which has an inlet and an outlet and which is bordered over an adequate area by a cooling surface in contact with a coolant fluid, the bled gas brushing against the cooling surface while passing through the cooling cavity from the inlet to the outlet. The coolant fluid may be ambient air, blown if necessary, but may equally be the lubricant or the fuel, which then acts as a heat-transfer fluid which has to be cooled itself, this most often being done using other coolers in contact with the ambient air.
The gas is then led to the items which are to use it by pipes. Cooling the turbine rotor is more tricky. The rotor disks usually have a radial cavity extending into each blade. The duct conveying the gas to the radial cavity passes through the center of the turbomachine near its axis of rotation and may consist of the turbine shaft itself.
Such devices for supplying pressurized gas exhibit numerous drawbacks. Firstly, bleeding gas from the compressor reduces the performance of the compressor, which has a direct impact on the thermodynamic performance of the turbomachine. Secondly, cooling the bled gas requires coolers which are placed outside the machine, these coolers generating drag in the case of turbine engines for aircraft. When use is made of an intermediate heat-transfer fluid; such as the lubricant or the fuel, effective regulation needs to be in place in order not to run the risk of carbonizing this heat-transfer fluid as a result of excessive heating. Finally, cooling the turbine rotor requires a complex and cumbersome circuit, part of which has to pass through the center of the turbomachine and interfere with the turbine shaft.
The invention addresses two problems, the first being that of reducing the fall in compressor performance, and the second being that of simplifying and lightening the cooling of the turbine rotor.
As a solution to the first problem the invention provides a turbomachine including a rotary compressor having an upstream end, a downstream end and means defining a flow path therebetween through which a stream of gas is compressed as it passes from said upstream end to said downstream end, said compressor including vanes disposed in said flow path and a compressor rotor having an outer wall which is contacted by said stream of gas as it flows along said flow path, and a pressurized gas supply device comprising a gas bleed orifice disposed in said rotary compressor and opening into said flow path thereof, and a cooling cavity disposed within said compressor rotor for cooling the pressurized gas bled from said gas stream via said gas bleed orifice, said cooling cavity comprising an inlet connected to said gas bleed orifice, an outlet, and a cooling surface for contact by said pressurized gas bled from said gas stream as said pressurized gas passes from said inlet to said outlet, said cooling surface being formed at least partly by at least a portion of the outer wall of said compressor rotor upstream of said gas bleed orifice whereby said bled pressurized gas is cooled by heat transfer through said outer wall of said compressor rotor to said gas stream upstream of said gas bleed orifice.
This arrangement has the result of returning to the gas stream, in the compressor, some of the energy which is bled off, thus reducing the drop in compressor performance resulting from the gas being bled off.
It will be understood that the transfer of heat from the bled gas back into the gas stream is made possible by the fact that this heat exchange takes place upstream of the gas bleed orifice, that is to say in a zone of the compressor where the gas stream is at a lower temperature than the temperature it has in the region of the bleed orifice. This exchange is made efficient by the presence of the rotor blades, which receive, by thermal conduction, some of the heat which is transferred to the outer wall of the compressor rotor, these blades returning the heat to the stream of gas by virtue of their large surface area which is swept at high speed by the gas flowing in the stream.
It will be understood that the cooling surface has to be large enough to allow the bled gas to be cooled, the person skilled in the art determining said heat-exchanger surface area according to the characteristics of the turbomachine, the point at which the gas is bled off, and the flow rate and temperature of the pressurized gas to be supplied.
The invention has the advantage of being simple to implement and of mainly using existing means, thus making it possible to reduce the mass and cost of the turbomachine.
Preferably, the cooling surface extends overall from downstream to upstream, that is to say that the bled gas sweeps the cooling surface from downstream to upstream. This arrangement has the effect of bringing the bled gas against zones of the cooling surface which, on the whole, are increasingly colder, and has the result of achieving a greater drop in the temperature of the bled gas. This arrangement also has the advantage of returning the heat to the gas stream uniformly along the compressor, which allows the operation of the compressor not to be disturbed.
It will be understood that all that is required in order to obtain the effect sought by the invention is for the flow of the bled gas in contact with the cooling surface to be generally in the direction from downstream to upstream. This effect will be maintained in spite of limited returns of bled gas in the downstream direction, it being possible for such returns to result from mechanical or aerodynamic constraints.
Preferably, the bleeding of gas is centripetal, the bleed orifice being located in the outer wall of the compressor rotor. This arrangement has the effect of shortening the path that the bled gas has to follow from the bleed orifice to the inlet of the cooling cavity, and has the result of allowing a short connection. In a preferred embodiment, the bleed orifice also constitutes the inlet to the cooling cavity, that is to say the bleed orifice opens directly into said cavity.
The turbomachine may have a plurality of cooling cavities in the compressor rotor, so as to be able to supply bled gas to different receivers and more or less independently of each other. In other words, the flow rate of pressurized gas supplied by one cooling cavity will have only a small influence on the temperature of the pressurized gas supplied by another cooling cavity.
The cooling cavities will preferably be arranged in the upstream-downstream direction, so as simultaneously to supply bled gas under different pressure and temperature conditions. Thus, a cooling cavity located toward the upstream end of the compressor will supply pressurized gas at a temperature and at a pressure which are lower than those for gas supplied by a cooling cavity nearer the downstream end of the compressor.
In the case of a turbomachine including a turbine having a rotor which is coaxial with the compressor and located downstream of said compressor and which is arranged to be cooled by the centrifugal passage of a flow of gas, the turbomachine is preferably provided with a tube disposed coaxially with the compressor and with the turbine, the upstream end of this tube being connected to the outlet of the cooling cavity and the downstream end of the tube being connected to the turbine rotor that is to be cooled. The function of this tube is to bring the bled pressurized gas directly to the turbine rotor from the cooling cavity, passing from upstream to downstream through the center of the turbomachine, that is to say near its geometric axis. This very simple arrangement makes it possible to make use of the central region of the turbine engine which is usually underused. It also allows the cooling gas to be conveyed through a straight short pipe of large cross section, which will minimize pressure drops.
Preferably, in the case of a turbine rotor with at least two stages, first and second stages A and B will be connected each to a respective one of first and second cooling cavities A and B by a respective one of first and second tubes A and B, the first turbine stage A being upstream of the second stage B, the first cooling cavity A being downstream of the second cooling cavity B, and the second tube B passing through the inside of the first tube A. An arrangement of this kind has the effect of conveying of the bled gases from the cooling cavities to the respective turbine stages that are to be cooled in the manner of concentric flows which do not cross, the flow of bled gas from the second cooling cavity B being conducted along the inside of the flow of bled gas from the first cooling cavity A. This arrangement thus allows the various flows of bled gas to be conveyed simply, and without crossing, from the cooling cavities which produced them to the turbine stages that are to be cooled. The arrangement also makes it possible to supply the various turbine stages with bled gas under temperature conditions suited to each stage: a stage located nearer the downstream end of the turbine, which is therefore not as hot, receiving bled gas from a cooling cavity located nearer the upstream end of the compressor, which is therefore also less hot. Finally, the arrangement makes it possible to maintain large passage cross sections which do not introduce pressure drops in the flow of bled gas.
The present invention is particularly effective when the compressor rotor is of the disk type, because the disks penetrate into the cooling cavity and increase the cooling surface area, which has the effect of increasing the ability of the cooling cavity to cool the bled gas.